Domain wall formation in minerals: I. theory of twin boundary shapes in Na-feldspar

Domains of triclinic Na-feldspar which are a direct consequence of structural strain are investigated theoretically. The following conclusions are arrived at. Only two types of twin walls are created by spontaneous strain, namely the Albite- and Pericline twin walls. If intersections occur, rounded corners are predicted. Any third domain wall close to such an intersection must bend into an S-shape. Narrow domains between two twin walls tend to become needle-shaped close to the intersection with a further, perpendicular, twin wall. The combination of the elastic stiffness coefficients c ₄₄ c ₆₆-c ₄₆ ² is expected to become small at temperatures near to the transition temperature. In the same temperature interval elastic fluctuations occur; their amplitudes and propagation directions are given.